1. Field of the Invention
This invention relates generally to electron guns and particularly is directed to an electron gun of uni-potential type with low spherical aberration.
2. Description of the Prior Art
Because an electron gun of uni-potential type has good blooming characteristic in the high electric current range, it is utilized in such devices as color picture tubes or projector tubes. In general, the electron gun of uni-potential type comprises a cathode K, a first grid (control electrode) G.sub.1, a second grid (acceleration electrode) G.sub.2 a third grid (first anode electrode)G.sub.3, a fourth grid (focusing electrode ) G.sub.4, and a fifth grid (second anode electrode) G.sub.5 arranged in this order. In this electron gun, in order that an electron beam may impinge with a smaller spot diameter on a phosphor screen surface, it is important to reduce as much as possible the spherical aberration of an electron lens, particularly a main electron lens formed of the third grid G.sub.3, the fourth grid G.sub.4 and the fifth grid G.sub.5. To this end it is required that an aperture diameter of each grid in the main electron lens system is made large. However, in order to make the grid aperture diameter large, it is necessary that the cathode ray tube envelope in which the electron gun is incorporated to have a neck portion of large inner diameter. However, the provision of a larger inner diameter of the neck portion lowers the deflection sensitivity of a deflection yoke.
On the other hand, as shown in FIG. 1, when the unipotential lens consists of a decelerating lens Lens 1 formed of the third and fourth grids G.sub.3 and G.sub.4, and an accelerating lens Lens 2 formed of the fourth and fifth grids G.sub.4 and G.sub.5, its electron lens action region can be separated, so that the aberration coefficient of the main electron lens system can be considered as being separated into the decelerating lens Lens 1 side and the accelerating lens Lens 2 side. Since the aberration coefficient is small in the decelerating lens and large in the accelerating lens, if the aberration amount of the accelerating lens is improved to have a further weaker lens action, the whole aberration amount of the uni-potential lens can be improved.
FIG. 2 shows an electron gun with low aberration coefficient we have previously proposed as a Japanese patent application No. 15581/1977 (no corresponding U.S. patent application), on the basis of the fact that the aforesaid aberration coefficient of the main electron lens system can be separated into the decelerating lens side and the accelerating lens side. This previously proposed electron gun comprises a cathode K, a first grid G.sub.1, a second grid G.sub.2, a third grid G.sub.3, a fourth grid G.sub.4 and a fifth grid G.sub.5 arranged sequentially in which an anode voltage V.sub.A is applied to the third and fifth grids G.sub.3 and G.sub.5 and a focusing voltage V.sub.F is applied to the fourth grid G.sub.4 permitting the third grid G.sub.3 to constitute a main electron lens system of unipotential type. In this electron gun, an electron lens diameter D.sub.1 of the front decelerating lens (Lens 1) forming the main electron lens system (namely, an aperture diameter of each opposing end of the third and fourth grids G.sub.3 and G.sub.4) is selected smaller than an electron lens diameter D.sub.2 of its rear accelerating lens (Lens 2) namely, an aperture diameter of each opposing end of the fourth and fifth grids G.sub.4 and G.sub.5) or to satisfy D.sub.2 &gt;D.sub.1, and the fourth grid G.sub.4 is made to have a length l=(l.sub.1 +l.sub.2) so as to be capable of separating the electron lens action region into those of the front and rear lenses Lens 1 and Lens 2 whereby the aberration coefficient of the main electron lens system can be made small. In the art, each of the grids G.sub.1 to grid G.sub.5 is held by a common insulation holding rod (so-called glass beads). Consequently, when the electron gun with the grids held together by the insulation holding rod is incorporated into the neck portion of the cathode ray tube envelope, the need for the space of the insulation holding rod restricts the diameter of an aperture of grid. When the electron gun is incorporated into the neck portion of, for example, 29 mm in inner diameter, the effective inner diameter of the grid is about 14 mm at best. In view of such aspect, we have previously proposed the electron gun shown in FIG. 2 capable of reducing the aberration coefficient by making the diameter of the declerating lens (Lens 1) small.